(1) Field of the Invention
The present invention relates to a superconducting magnet which uses a container having a diametrically crossing beam member which supports a coil container containing a superconducting coil and, especially to a preferable superconducting magnet for avoiding quenching by reducing heat generation which is caused by the beam member installed in a dynamic environment.
(2) Background of the Invention
A schematic perspective cross section of a superconducting magnet having a beam member with a coil container is indicated in FIG. 2. The numeral 1 is a superconducting coil, 2 is a ring-shape superconducting coil container which contains such a coolant as liquid helium and the superconducting coil 1, 3 is a beam member crossing a diametrical portion of the superconducting coil container 2, 4 is a radiant heat shield for protecting the container 2 from entering the radiant heat by covering an outer surface of the container 2, 5 is a vacuum adiabatic vessel, and 6 is a supporting member for fixing the superconducting coil vessel 2 to the vacuum adiabatic vessel 5. Generally speaking, as materials for the superconducting coil container 2, such materials as stainless steel (hereinafter shortly called SUS) having high stiffness and strength, etc., are used in order to support hoop stress of the superconducting coil 1. Similarly, SUS is used as material for the beam member 3 and the supporting member 6 both of which support electromagnetic force and heavy weight. On the other hand, as materials for the radiant heat shield 4, aluminum which has high radiation reflectance, light weight, and large thermal conductivity etc. are generally used. As materials for the vacuum adiabatic vessel 5, SUS and other thick wall materials are generally used in order to maintain inside of the vessel vacuum for protection from inflow of external heat and to support the superconducting coil etc.
Superconducting magnet causes quenching when temperature of superconducting wiring material composing the superconducting coil 1 is elevated higher than the critical temperature of the material, and come to be unable to maintain the superconducting state. Accordingly, it is an important issue to maintain temperature of the superconducting coil 1 lower than the critical temperature, and to keep the superconducting state.
Hitherto, as for inflow of external heat, countermeasures using the above described radiant heat shield 4 and the vacuum adiabatic vessel 5 etc. have been taken for preventing radiation and heat transfer. Further, various countermeasures against inflow of external heat by conduction, for example, such a method by extending heat flow path as disclosed in JP-A-57-208111 (1982) have been taken.
The above described countermeasures for preventing inflow of external heat is premised on an assumption that the superconducting magnet is used in a static environment. Accordingly, any countermeasures are not considered on heat generation in an internal operation of the superconducting magnet itself when, for example, any external force is added to the superconducting magnet, or the superconducting magnet is used in a dynamic environment. One of sources which generates heat in the case when the superconducting magnet is used in a dynamic environment is eddy current which is generated in the superconducting coil container. A conventional superconducting magnet has a structure shown in FIG. 2 wherein the superconducting coil container 2 is directly fixed to the vacuum adiabatic vessel 5 with the supporting member 6, and the radiant heat shield 4 is fixed to the supporting member 6 which supports the superconducting coil container 2. Moreover, conventional radiant heat shields are generally made of aluminum, and have a structure which easily causes relative vibration to the superconducting coil 1 because of thinness and light weight. Therefore, when vibration is transmitted inside from outside, relative vibration between the superconducting.sub.G coil and the radiant heat shield is caused, and the radiant heat shield 4 comes to be crossing a strong magnetic field which is yielded by the superconducting coil 1. Accordingly, eddy current is induced in plate members of the radiant heat shield 4, further eddy current is induced in the superconducting coil container 2 by crossing over the superconducting coil container 2 by magnetic field induced by the above described eddy current, and, consequently, the eddy current becomes a cause of heat generation and causes a problem to make the superconducting coil become quenched.
As for the above described problem, a countermeasure to suppress the heat Generation by adhering low resistant material such as aluminum, etc., on the superconducting coil container 2 in order to flow the eddy current through the low resistant material even if the eddy current is caused as disclosed in JP-A-60-217610 (1985).
But, hitherto, investigation whether the above described disclosure effects on heat Generation of the beam member 3 which is fixed to the superconducting container 2 or not, and if the effect may exist, how much does it effect on the heat Generation of the beam member 3 has not been performed, and, consequently, none of countermeasures for heat Generation of the beam member 3 is considered.